Optical communication systems may use several forms of data or information multiplexing, including frequency-division multiplexing, wavelength-division multiplexing and time-division multiplexing. These various forms of information multiplexing have been explored extensively and are rapidly approaching a point of diminishing returns in terms of bandwidth in optical fibers; however, the demand for increased bandwidth continues to grow. Therefore, new methods of increasing optical fiber capacity are being explored.
Spatial domain multiplexing (SDM) and orbital angular momentum multiplexing (OAM) have been studied for use in increasing the bandwidth of optical fibers. SDM is a multiplexing technique that adds a new degree of photon freedom inside the fiber and allows for a multifold increase in communication bandwidth. SDM allows co-propagation of multiple channels of the same wavelength, allowing spatial reuse of optical frequencies inside a single core. Another version of SDM allows for multiple cores inside of a single cladding, which is akin to laying down more optical fibers, albeit with a better form factor. The growth rate of data usage today requires that more and more fibers be laid down to cope with growing needs.
In short, SDM technology is a multiplexed communication system that allows for transmission of independent data channels inside a single fiber. SDM produces spatially separated co-propagating spatial channels as a function of input angle of each optical channel. Each channel's input angle results in a separate data channel propagating on an independent helical traveling path inside the carrier fiber. The architecture of an exemplary SDM system is shown in FIG. 1. As shown, sources 105 and 110, which may be, for example, pigtail laser sources, launch optical energy (or light) into single mode input fibers 115 and 120. Beam combiner module 125 ensures the relative positioning of input fibers 115 and 120 and multimode carrier fiber 130 so that the optical energy from input fiber 115 and the optical energy from input fiber 120 are launched into carrier fiber 130 at different angles. The range of the input angles of the optical energy into carrier fiber 130 can be as large as allowed by the numerical aperture of carrier fiber 130, with larger angles providing wider helical trajectories inside the fiber. Optical energy from each of sources 105 and 110 traverses the entire length of carrier fiber 130 following separate helical paths. The electric field at the center of these helically propagating waves becomes negligible, due to optical vector vortices allowing for co-propagation of the same wavelength with limited interference or crosstalk. The helically propagated optical energy exits carrier fiber 130 in the form of concentric circular rings which make up SDM intensity profile 135. Each of the rings of SDM intensity profile 135 represent a channel of optical energy. SDM intensity profile 135 is spatially de-multiplexed by beam separator module 140 which may comprise combination of lenses 145. Beam separator module 140 routes the optical energy from the individual channels to photodetectors 150 and 155.
An exemplary SDM intensity profile of a three-channel SDM system is shown in FIG. 2. The exemplary SDM intensity profile comprises three rings, one for each source/channel in the SDM system. The intensity of each source/channel is reflected by the respective ring for that source/channel. The challenge in an all-optical detector design involves capturing photons from the SDM intensity profile output and efficiently guiding them into subsequent fibers or pigtail detectors.
In a typical fiber optic system, the optical energy from a single source/channel that is output from a carrier fiber is read with a PIN diode, which is illustrated in FIG. 3. Incident light 205, from optical fiber 210 is passed to PIN diode 215. Incident light 205 will induce electron-hole pairs in PIN diode 215 causing current to flow relative to the intensity of incident light 205.
A single PIN diode, such as PIN diode 215, does not work well in SDM systems. The incident light from an optical fiber carrying multiple SDM signals would induce only a single current flow in the PIN diode, defeating the purpose of SDM. One method to read each channel of the signal is to use a PIN diode having an octagonal shape. One such design using an array of complementary metal oxide semiconductor (CMOS) photodiodes was described in U.S. Pat. No. 8,278,728 to Murshid, et al. Each region in the array design has a separate p- and n-region. Each of these regions can be connected to separate loads to read each channel of the SDM signal independently.
The CMOS photodiode design de-multiplexes the SDM signal; however, it requires an optical-to-electrical (O/E) conversion. O/E conversions often increase the system complexity and typically limit the bandwidth that can be obtained from a particular signal or channel. This limits the potential usefulness of systems using O/E conversion in communication systems. In addition, use of CMOS photodiode design in current systems requires complete recertification of the system, which is a lengthy and costly process. Therefore, an optical-to-optical solution is needed that will provide for greater bandwidth.